The present invention relates to novel polymers that are based on a cellulose backbone and grafted with a controlled number of grafts of controlled length. These novel polymers are prepared by radical polymerization techniques, which can control the architecture of the polymer.
The grafting of synthetic polymers onto a cellulosic backbone has been the subject of research activities for a long time. The hope is to capture the benefits of a polymer that has properties of both cellulose and the synthetic polymers. Enormous research and development efforts have occurred over the last 40 years, but no commercializable polymer or process has yet been discovered, despite optimistic predictions.
The grafting of polymers on a cellulosic backbone proceeds through radical polymerization wherein an ethylenic monomer is contacted with a soluble or insoluble cellulosic material together with a free radical initiator. The radical thus formed reacts on the cellulosic backbone (usually by proton abstraction), creates radicals on the cellulosic chain, which subsequently react with monomers to form graft chains on the cellulosic backbone. Related techniques use other sources of radical such high energy irradiation or oxydising agents such as Cerium salt, or redox system such as thiocarbonate-potassium bromate. These method are well known, see, e.g., Mc Donald, et al. Prog. Polym. Sci. 1984, 10, 1; Hebeish et al. xe2x80x9cThe Chemistry and Technology of cellulosic copolymersxe2x80x9d, (Springer Verlag, 1981); Samal et al. J. Macromol. Sci-Rev. Macromol. Chem, 1986, 26, 81; Waly et al, Polymers and polymer composites 4, 1, 53, 1996; and D. Klenn et al., Comprehensive Cellulose Chemistry, vol. 2 xe2x80x9cFunctionalization of Cellulosexe2x80x9d pp. 17-31 (Wiley-VCH, Weinheim, 1998); each of which is incorporated herein by reference.
Another strategy involves functionalizing the cellulose backbone with a reactive double bond and polymerizing in presence of monomers under conventional free radical polymerization conditions, see, e.g., U.S. Pat. No. 4,758,645. Alternatively a free radical initiator is covalently linked to the polysaccharide backbone to generate a radical from the backbone to initiate polymerization and form graft copolymers (see, e.g., Bojanic V, J, Appl. Polym. Sci., 60, 1719-1725, 1996 and Zheng et al, ibid, 66, 307-317, 1997). For example, in U.S. Pat. No. 4,206,108, a thiol is covalently bound to a polymeric backbone with pendent hydroxy groups via an urethane linkage; this polymer containing mercapto groups is reacted with ethylenically unsaturated monomers to form the graft copolymer.
However, none of these techniques lead to a well-defined material with a controlled macrostructure and microstructure. For instance none of these techniques lead to a good control of both the number of grafts chains per cellulose backbone molecule and molecular weight of the graft chains. Moreover side reactions are difficult, if not impossible, to avoid, including the formation of un-grafted polymer, graft chain degradation and/or crosslinking of the grafted chains.
To solve these problems, pre-formed chains have been chemically grafted onto cellulosic polymers. For instance, in U.S. Pat. No. 4,891,404 polystyrene chains were grown in an anionic polymerization and capped with, e.g., CO2. These grafts were then attached to mesylated or tosylated cellulose triacetate by nucleophilic displacement. This method is difficult to commercialize because of the stringent conditions required by the method. Moreover, the set of monomers that can be used in this method is restricted to non-polar olefins, namely precluding any application in water media.
Block copolymers based on cellulose esters have been reported. See, e.g., Oliveira et al, Polymer, 35, 9, 1994; Feger et al, Polymer Bulletin, 3, 407, 1980; Feger et al, Ibid, 6, 321, 1982; U.S. Pat. No. 3,386,932; Steinmann, Polym. Preprint, Am. Chem. Soc. Div. Polym. Chem. 1970, 11, 285; Kim et al., J. Polym. Sci. Polym. Lett. Ed., 1973, 11, 731; and Kim et al., J. Macromol. Sci., Chem. (A) 1976, 10, 671, each of which is incorporated herein by reference. A major problem with these references is the generation of considerable chain branching, grafting or crosslinking. Mezger et al., Angew. Makromol. Chem., 116, 13, 1983 prepared oligomeric monohydroxy-terminated cellulose coupled with 4-4xe2x80x2diphenyl-disocyanate, which was then used as a UV-macro-photo-initiator to prepare triblock copolymers. The reaction is known as the iniferter technique and uses UV initiation, which limits its applicability to certain processing methods and furthermore is typically applicable to styrenic and methacrylic monomers. Other monomers, such as acrylics, vinyl acetate, acrylamide type monomers, which are in widespread use in waterborne systems, might require another technique.
Previously, it has been recognized in the art that cellulose based materials adhere to cotton fibers. For example, WO 00/18861 and WO 00/18862 disclose cellulosic compounds having a benefit agent attached, so that the benefit agent will be attached to the fiber. See also WO 99/14925. However, the ability of cellulose based materials to adhere has not been fully investigated, and a need exists to find cellulosic based materials that are of commercial significance.
Therefore, there is a strong need to develop a process that makes it possible to prepare either block or grafted materials from cellulosic polymers, with a predictable number of blocks or graft chains per cellulosic backbone in a waterborne system. These blocks and graft chains should be controlled in length and chemical composition. Moreover, the method of synthesis should be commercializable. Furthermore, a need exists to provide benefits to fibers and surfaces.
This invention solves, at least in part, these needs by providing a process that can be implemented under conditions similar to conventional polymerization, which is applicable to a large of variety of hydrophilic and hydrophobic monomers. This invention provides a living or controlled free radical method of preparing cellulosic graft polymers by attaching a free radical control agent to a controlled number of sites on a cellulose backbone, where the cellulose backbone has been sized to a desired degree of polymerization. The grafts are then grown to a desired size using living-type kinetics, with the grafts being chosen from a wide variety of one or more monomers. When the grafts are located at one or more terminal end portions of the cellulosic backbone, then the polymers are considered herein to be block copolymers.
The cellulosic grafted and copolymeric materials of this invention with well-defined macromolecular features find utility in a wide field of applications. In particular, due to their segmented structures, these polymers have applicability as compatibilizers between naturally occurring bio-polymers such as starch or cellulose with synthetic thermoplastic resins, so-called biodegradable bio-plastics.
Furthermore, the polymers of this invention may be water soluble, or at least water-dispersible (e.g., water swellable). In some of these embodiments, the cellulosic moiety is known to adsorb to cellulosic surface, such as cotton or paper, which then alter the surface or interface of cotton/paper and bring new benefits to the fiber or surface.
The process of this invention has a number of benefits, which can be considered objects of this invention, including (1) control of the molecular weight of the cellulosic backbone through the depolymerization process, (2) little or no significant side reactions that leads to crosslinking or chain severing of the cellulosic backbone, (3) control of the grafting site density, (4) control of the graft or block length, (5) minimization of ungrafted material, and (6) control of the graft or block chemical composition (e.g., high chain homogeneity as compared to conventional free radical processes).
It is another aspect and object of this invention to provide cellulosic backbone polymers that have a controlled degree of graft substitution.
It is an object of this invention to prepare cellulosic graft polymers by growing grafts with living kinetics from a cellulosic backbone.
It is another object of this invention to provide a method of preparing cellulosic graft polymers.
It is yet another object of this invention to provide cellulosic graft polymers that adhere to fibers or surfaces, preferably in the presence of water.
It is a further object of this invention to modify a cotton or paper fiber or surface in an aqueous treatment step.
It is still a further object of this invention to modify a cotton or paper fiber or surface with a cellulosic graft polymer having known properties.
Further aspects and objects of this invention will be evident to those of skill in the art upon review of this specification.